In general, gastrointestinal (GI) motility disorders can affect any part of the GI tract, from the oesophagus to the colon. The clinical impact of GI motility disorders in small animal ICU patients is unknown but likely significant. Contributing to the ambiguity of these disorders, particularly ileus, is the lack of evidence regarding the prevalence of these disorders in dogs and cats, controversy over efficacy of GI prokinetic drugs in small animals, and the challenges in clinically diagnosing the conditions. Although oesophageal and colonic motility disorders can occur in hospitalized ICU patients these conditions are more likely to exist prior to the patient being admitted to the ICU, and not as a result of ICU hospitalization. The majority of this presentation will therefore focus on conditions that are more commonly encountered in the ICU and which are associated with hospital interventions, treatments and stresses.
Etiology of GI Motility Disorders
GI motility involves a series of complex coordinated events that store, grind and propel ingesta through the GI tract in such a way that absorption, fermentation, water reabsorption and elimination of unwanted metabolites can take place. The complex coordinated control of GI motility is a function of smooth muscle contraction regulated by myogenic, neural, and hormonal factors (readers are referred to other review articles on the subject). Given the complexity of regulation and coordinated efforts responsible for GI motility, there are a number of disorders that can contribute to GI dysmotility.
Functional Small Intestinal Ileus and Delayed Gastric Emptying
Ileus is defined as a transient cessation or abnormal pattern of gastrointestinal (GI) motility. Some of the more common general causes of ileus include postoperative ileus, dysautonomia, muscular dystrophy, idiopathic pseudo-obstruction (intestinal leiomyositis), viral enteritis and hypothyroidism. Although the exact incidence is unknown in small animal patients, postoperative ileus is the most common ICU cause in human ICU patients, with a prevalence (depending on the definition used) as high as 25% following elective surgery. Principal underlying causes of postoperative ileus in ICU patients are believed to be the result of electromechanical dissociation of the intestinal musculature due to increased sympathetic tone, release of humoral inhibitory factors (catecholamines, vasopressin, endogenous opiates), impaired release of prokinetic hormones (neurotensin, motilin), and/or hypokalaemia. General conditions in both human and canine ICU patients associated with postoperative ileus include anaesthesia, analgesics (particularly opioids), prolonged surgery time, fasting, and surgery induced inflammation.
Postoperative ileus is known to cause vomiting, decreased tolerance of oral diets, prolonged recovery from surgery, and increased morbidity and mortality in human ICU patients. In addition to the discomfort reported in people, postoperative ileus may predispose patients to wound dehiscence, intussusception and pulmonary and thromboembolic complications. Therefore, efforts should be made to rapidly identify the condition and to treat when it is identified. Despite the importance of recognizing ileus during the postoperative period in people, the veterinary literature is sparse regarding the diagnosis and treatment of postoperative ileus in dogs and cats.
Diagnosis of Ileus and Delayed Gastric Emptying
There are several invasive techniques to evaluate changes in GI motility which include serosal electrodes, strain-gauge force transducers, and wireless motility capsule devices. Scintigraphy is arguably the current gold standard for dogs and cats. Serosal electrodes and strain-gauge force transducers require surgical implantation of electrodes. Wireless motility capsule devices require oral administration and monitoring until they are expelled from the body, which can take several days and tend to be limited to patients >15 kg. A more practical approach in the hospitalized clinical patient is ultrasound.
Ultrasound Evaluation of GI Contractions
Transabdominal sonographic techniques have been used in both dogs and people to diagnose changes in GI motility. Abdominal ultrasound is safe and noninvasive and requires minimal patient preparation, making it a practical diagnostic tool in the clinical setting. The normal GI peristalsis in dogs is reported to be 4–5 contractions per minute in the stomach and proximal duodenum and 1–3 contractions per minute in the jejunum. Fasting and stress of hospitalization have been shown to decreases GI motility in dogs and should be considered in ICU patients.
Ultrasound Evaluation of Gastric Emptying Time
In addition to determining the presence and/or absence of GI contractions, qualitative evaluation of GI motility using ultrasonography can also be performed. Ultrasound has been shown to be a reliable and accurate quantitative method to measure emptying of liquids and solids from the stomach. Dogs are gently restrained while standing and the ultrasound transducer is placed in a longitudinal orientation on to the ventral midline, caudal to the xyphoid. Electronic calipers are used to measure the craniocaudal and ventrodorsal diameters of the gastric antrum. The antral area is calculated using the software incorporated in the ultrasound machine. Measurements are made at each of the following each time points: 30 min prior to feeding, then every 20 min post-prandially for 6 hours. Gastric half-emptying time (50% of AUC), time of maximal antral area, and time at which antral area was reduced by 50% can be calculated. This method is easily applicable to clinical patients but requires the availability of an experienced ultrasonographer.
Treatment
Therapy of functional, nonobstructive disorders of gastric motility is based on identification and correction of factors contributing to decreased GI motility and promotion of GI motility through environmental changes, dietary modification, and judicious use of prokinetic drugs. Feeding soon after surgery should be implemented unless otherwise contraindicated. Liquid diets have more rapid gastric emptying than solid foods and are often chosen in early postoperative phases. Medium calorie diets and low-fat diets also been reported to have shorter gastric emptying times. Given there is some evidence to suggest an increased number and smaller size meals may also be helpful in promoting more rapid gastric emptying, consideration of a continuous rate infusion liquid diet may prove beneficial, although there is no clinical evidence to support this theory. Other recommendations include early ambulation, epidural analgesia (decreases the need for systemic opioid analgesia which is often associated with ileus), nonsteroidal anti-inflammatory agents and minimally invasive surgery.
If gastric emptying is significantly delayed and a fluid filled stomach is noted, some clinicians prefer to place a nasogastric tube to allow gastric emptying which may relieve patient discomfort, improve gastric motility and decrease the risk of aspiration. Gastric emptying is often used in conjunction with continuous rate nasogastric tube infusions or bolus liquid diet nasogastric tube feeding.
The available published evidence on the effect of various prokinetic drugs in healthy dogs and cats is tenuous, while it is almost nonexistent for dogs and cats with functional ileus. Some of the more common prokinetic drugs that have been studied in small animals include metoclopramide, cisapride, erythromycin, azithromycin, and ranitidine.